34 -Bevel Wheel Moulding. proper radius of arm is -fixed,"by lowering the tooth pattern, rilling up with sand, raising, rotating the amount of the pitch, lowering again, and so on until the whole wheel is formed. The. machine is now taken away by attaching the crane chain to the eye-bolt at the top of central spindle (see also bottom off. 58). Core boxes are needed for the wheel boss and segments between the arms, and for-central hole. Wood strips are used as gauges to fix the cores in position and to preserve the proper thickness of the rim and arms; and a cope being placed over all, gates are formed and the wheel cast. Bevel wheels are moulded in a similar manner to the above, the principal alterations being the strickle boards and tooth patterns. Fig. 48 will make this clear. A board A strikes out the back of the wheel in green sand, and parting sand is applied; an impression of the wheel back is then taken in the top box, and the latter removed to finish; board B next forms bottom face, cores and boss pattern completing the remainder. The core- boxes for the wheel arms are shewn in 72^, p. 6t, and similar boxes will be found in Fig. 42.* ^ Chilled Castings.—Where a very hard and durable surface is required to a casting, u chilling " is effected by making that part of the mould, where the said face occurs, of iron. When the molten metal meets the surface of cold iron, it cools rapidly and forms crystals of white cast iron, hard yet brittle, where it meets the iron mould, and, fpr a .depth of* ,an inch gr> rriore within the casting, according to the mixture used, or' the weight of the chill mould; the rest of the casting is still grey and soft. It would seem that the graphite crystals do not, under such circumstances, have time to form, and so the carbon becomes ! combined with the iron. Suitable objects for chill casting are:—rolls for plate mills used in forging plates (see Fig. 275, p. 280), and which require a great depth of chilling, as they have to be trued up again on being worn down; tram-car wheels, the rim only being chilled, this class of traffic not being capable of supporting the expense of steel tyres; points of plough-shares, which wear away at a very rapid rate in the earth; bushes for ordinary cart axles; railway chairs; pro- * For other moulding machines see Appendices II., p. 785, and V., p. 969.